The need to have a torsion box constructed on a plane surface to prevent the potato chip outcome presents challenges, but the one scheme that appealed to me was the cross sting thing. To test that out, I constructed an experiment using my I-beams [which are pretty straight by the way] and a couple of adjustable horses. The photos are pretty much self explanatory but suffice it to say that they need to just kiss in the middle of the whole deal without one actually distorting the other. To do that with standard horses and wedges could be a challenge, so I used a clamp in opposing configuration to accomplish it on my adjustable horses.

Needless to say: The stings need to be tight and lifted from the surrounding stuff to make the deal work. I stretched the string and used blocks of wood cut from the same stick to lift the strings above the I-beams. It was almost perfect but using the opposing jaw clamp to lift a corner to just make them kiss confirmed that the I-beams are in the same plane.